It is known that, prior to the landing on a runway, an airplane must follow an approach trajectory that ends by a stabilized final approach. On such approach trajectory, the airplane decelerates (minimum engine thrust) to reach a setpoint approach speed at a stabilization point. During the approach, the pilot controls the different aerodynamic configurations, as well as the extension of the landing gear. The ideal case is the one when the airplane reaches at the stabilization point with a speed being substantially close of the setpoint approach speed and in a landing configuration. Upon such stabilized final approach, the airplane in a landing configuration follows a final approach axis (also denoted “glide”) with a slope with respect to the ground being predefined (generally −3°), with the setpoint approach speed, an adjustment of the engine speed to maintain said approach speed and a stabilized trim from a setpoint stabilization altitude (associated with said stabilization point), for example, equal to 1000 feet (about 300 meters).
However, it may occur that the final approach is a non stabilized approach due to too a short distance to the runway to dissipate the energy, thereby preventing to join the stabilisation altitude in stabilized flight conditions (case of over-energy) or due to a quick loss of energy (case of under-energy) or still from events external to the aircraft, leading to approach trouble.
Moreover, it is known that there are presently different actions allowing the airplane to be restored on a stabilized approach in the case when an over- or under-energy state of the latter is detected.
In particular, in the case of a non detection of an under-energy state of the airplane, i.e. when the fineness of the airplane is too much downgraded (aerodynamic configurations being established too soon, front wind, weak speed and the airplane far from the runway, etc.), the airplane will follow the approach trajectory with a reduction of its speed so that the setpoint approach speed will be reached well before the stabilization point (located on the final approach axis at the stabilization altitude, for example at 3 nautical miles from the runway threshold for a final approach axis of −3 degrees).
The word “fineness” of the airplane means the aerodynamic efficiency of the airplane. So, the total energy variation of the airplane depends on the fineness and on the thrust of the airplane engines.
In such a situation, the under-energy state is only detected lately and the pilots use the engines sooner (in comparison to the ideal case) to maintain the setpoint approach speed up to the stabilization point, and then until the runway threshold.
However, a later detection of an under-energy state leads to a necessary use of the engines, thereby causing:                an extra fuel consumption;        engine wear and tear; and        noise in the cabin and at the level of the ground.        
When the under-energy state is detected sufficiently soon with the help of a prediction device for the energy state of the airplane, the pilots can decide to maintain the current speed by using the engines, thru an increase of the engine rating. The fineness of the airplane will then not be downgraded so much. Once the pilots consider as necessary to reposition the engines at the idle speed, the airplane can continue its deceleration so as to reach the approach speed at the stabilization point.
However, even in this last case, an anticipated use of the engines leads to:                an extra fuel consumption;        engine wear and tear; and        noise in the cabin and at the level of the ground (although the latter is reduced with respect to the preceding case, as the airplane is higher).        
Furthermore, in the case of a detection of an over-energy state of the airplane, i.e. when the fineness of the airplane is not enough downgraded (aerodynamic configurations not yet established, rear wind, too rapid airplane and too close to the runway, etc.), the airplane will follow the approach trajectory with a reduction of its speed such that the approach speed will be reached well before the stabilization point.
In such situation, the over-energy state is only detected later and the pilots use the airbrakes so as to increase the speed reduction capacities up to the runway threshold.
However, a later detection of the over-energy state leads to a necessary use of the airbrakes, thereby leading to:                the non respect in certain cases of the approach stabilization procedure;        possibly a cancellation of the approach, being followed by a go-around;        noise in the cabin and at the level of the ground; and        discomfort for the passengers.        
When the over-energy state is detected soon enough with the help of a prediction device for the energy state of the airplane, the pilots can decide to extend anticipatively:                either the airbrakes to downgrade more the fineness of the airplane. Once the pilots consider as necessary to retract the airbrakes, the airplane can continue its deceleration so as to reach the setpoint approach speed at the stabilization point;        or the slat and flap configurations so as to downgrade the fineness of the airplane more. The airplane will then decelerate more up to the setpoint approach speed at the stabilization point;        or the landing gear to downgrade more the fineness of the airplane. The airplane will then decelerate more up to the setpoint approach speed at the stabilisation point.        
However, an anticipated use of the airbrakes, the slat and flap configurations or the landing gear will cause noise in the cabin and at the level of the ground and possibly discomfort for the passengers.
In short, the use of the different above-mentioned means (engines, airbrakes, slat and flap configuration, landing gear) to restore a stabilized approach can generate:                an extra fuel consumption;        engine wear and tear;        noise in the cabin;        noise at the level of the ground;        discomfort for the passengers.        